1. Field of the Invention
The present invention is a method and apparatus for polling transmission status in a wireless communications system, in particular a method and apparatus for avoiding a transmitter being unable to successfully execute a polling procedure to a receiver and avoiding the transmitter executing unnecessary polls.
2. Description of the Prior Art
With the coming of the information age, requirements of mobile voice, data communications, and various mobile services increase each day. Communications systems of prior art are facing bottlenecks of channel usage overflowing and inefficient transmission speeds. Therefore, the third generation mobile communication was developed to have better usage of the frequency spectrum and higher-speed transmission service. Compared to the existing second generation mobile communication technology, the most significant difference with the third generation mobile communication is that the third generation mobile communication uses a Wideband Code Division Multiple Access (WCDMA) protocol for wireless access. The WCDMA protocol provides a more efficient channel usage, outstanding coverage, and high quality, high-speed multimedia data transmission. The WCDMA protocol can satisfy different service requirements of QoS, provide flexible duplex transmission services, and also better communication quality.
With a set of protocol specifications developed by the 3rd Generation Partnership Project (3GPP), the Universal Mobile Telecommunications System (UMTS) can provide different levels of transmission quality and operate in different modes according to the different requirements, for instance, Transparent Mode (TM), Unacknowledged Mode (UM), Acknowledged Mode (AM). TM is used for services requiring instant transmission, UM is used for services requiring instant transmission as well as packet orders, and AM is used for robust services requiring data accuracy but not instant transmission.
In AM, for better data accuracy and robustness, Radio Link Control (RLC) layers of a transmitter and a receiver initiate a status report procedure at an appropriate time to have the transmitter poll the receiver so that the receiver transmits a status report unit, or a piggybacked status report unit to the transmitter. The transmitter then executes appropriate procedures according to the status report unit from the receiver. As the transmitter polls the receiver, the transmitter selects a Protocol Data Unit (PDU) and sets a polling bit of the PDU as ON (=1) which requests the receiver to transmit a status report unit. Please refer to the following descriptions for a method for polling (i.e. for carrying an ON polling bit) of a PDU in the prior art.
As stated in the specification 3GPP TS 25.322 V6.3.0 (2005-03), when a poll is triggered and polling is not prohibited and no PDU is scheduled for transmission or retransmission, a PDU that has been transmitted and not yet acknowledged is selected to poll the receiver. Now, if a transmitting window size of the transmitter is larger than or equal to 2048, then a PDU with a sequence number (SN) equal to an SN of a next PDU to be transmitted for the first time minus 1 is selected, and the selected PDU is scheduled for retransmission after setting its polling bit. In contrast, if the transmitting window size of the transmitter is smaller than 2048, then the prior art can select a PDU with an SN equal to an SN of a next PDU to be transmitted for the first time minus 1 or a PDU that has not been acknowledged, and the selected PDU is scheduled for retransmission after setting its polling bit.
The transmitter is able to request the receiver to transmit a status report unit with polling, in order to get the status of PDU receiving by the receiver. With the receiving status, the transmitter decides whether to retransmit PDUs to maintain data robustness. To keep transmission efficient, it is significant to efficiently select a PDU to poll.
On the other hand, in UMTS, if the transmitter transmits a Service Data Unit (SDU) and the SDU is retransmitted for a predetermined number of times unsuccessfully, or the transmitter never receives a positive acknowledgement corresponding to the SDU from the receiver after a predetermined period of time, for better transmission efficiency, the transmitter (with an explicit signaling procedure) discards the SDU and transmits a Move Receiving Window (MRW) super-field to the receiver. After the transmitter discards the SDU, PDUs that carry segments of the SDU and/or the Length Indicator indicating the ending position of the SDU are also discarded. Therefore, the transmitter can improve transmission efficiency to reduce system resources wasted on unnecessary retransmission of PDUs and waiting for acknowledgements.
In some cases, all PDUs stored in a buffer of the transmitter are discarded before being acknowledged, and an error occurs when selecting a PDU to poll. For example, assuming the transmitting window size of the transmitter is 4, and PDUs with SN=0, 1, 2, 3 are transmitted by the transmitter. If the PDUs with SN=0, 1, 2, 3 have been retransmitted for many times or a long time has been elapsed waiting for positive acknowledgements of the PDUs, a discarding procedure of the PDUs is initiated, and the PDUs with SN=0, 1, 2, 3 are discarded from a buffer of the transmitter which means that there are no PDUs in the buffer of the transmitter. Now, assume that there are no PDUs scheduled for transmission, and a poll is triggered and polling is not prohibited. Since the transmitting window of the transmitter is smaller than 2048, the transmitter can select a PDU with an SN equal to an SN of a next PDU to be transmitted for the first time (=4) minus 1 (i.e. a PDU with SN=3), or select a PDU that has not been acknowledged (i.e. any one PDU among PDUs with SN=0, 1, 2, 3), to be a PDU for polling. However, because the buffer of the transmitter has no PDUs inside, an error occurs since the transmitter cannot successfully select a PDU for polling.
In summary, in the prior art, when all PDUs in the buffer of the transmitter are discarded before being positively acknowledged, the transmitter cannot successfully execute a polling operation, which leads to system errors. In fact, in this case, the transmitter does not need to poll the receiver. Therefore, an extra operation lowers an overall transmission efficiency and induces system resource wastage.